fix(sys-apps/systemd-sysv-utils): Change install location, not target.

Change the install location based on symlink-usr instead of the target
and use absolute symlinks. We need to move towards never installing to
/{bin,sbin,lib...} and the use of absolute links avoids needlessly
breaking if the symlink-usr flag and the current state of /bin don't
agree (i.e. between flipping the flag and migrating the current files).